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Knighton et al (1991) have documented dramatic changes to the lower Mary River floodplains associated with upstream expansion of the dendritic tidal creek network since 1950. Tidal creek expansion has resulted in the reimposition of a saltwater influence on the floodplains (Knighton et al 1991). Saltwater has invaded low-lying freshwater wetlands, destroying the associated vegetation and causing dieback of large areas of Melaleuca (paperbark) spp.

From a comparison of 1950 and 1983 aerial photography, Woodroffe et al (1986), Fogarty (1982) and O’Neil (1983) identified evidence of recent tidal creek extension and saltwater intrusion on the South Alligator River floodplains in the Alligator Rivers Region. Several tidal creeks have extended headward towards freshwater wetlands of the floodplains. Mangroves have encroached on creeks that have become more tidally active. Areas of upper intertidal higher level mudflats have expanded apparently due to more frequent tidal inundation. Woodroffe et al (1986) noted areas of dead Melaleuca spp swamp as evidence of saltwater intrusion into freshwater billabongs and swamps via the extending creeks.

Similar observations of Melaleuca spp dieback have been observed on the Magela Creek system of the East Alligator River (Williams 1984). From comparison of aerial photographs taken in 1950 and 1976, 38% of the perennial freshwater forest, dominantly Melaleuca spp that covered almost 60% of the floodplain in 1950, suffered significant loss. The changes in Melaleuca spp forest density was attributed to factors other than plant succession and sediment accumulation in the swamp, although saltwater intrusion was not specifically identified as a causal effect. Lowry & Riley (2004) subsequently found a further 21% decrease in the overall density of Melaleuca spp between 1976 and 1996.

2 Factors contributing to saltwater intrusion Several very large magnitude physical factors determine biophysical changes on the floodplains and contribute substantially to the estuarine processes driving intrusion of salt water into the coastal lowlands and formation of the coastal plains. They include variability of climate, fluctuation in sea level, stream hydrology and morphology of the coastal plain. In turn, these contribute to secondary processes affecting the stability of tidal creeks, such as the distribution of vegetation communities and human use of the coastal plain. All factors require further field survey and closer examination. In particular, evaluation and modelling of the fresh and saline water interface requires a well surveyed long profile as well as surveyed channel cross-sections to identify channel width and depth in a manner similar to the surveys conducted by Vertessy (1990) in the South Alligator River for numerical modelling and a fuller appreciation of processes affecting the development of tidal creeks.

2.1 Wind and weather Three features of the regional climate are of direct relevance to the hydrology of tidal streams: the seasonal rainfall pattern; changes in wind direction, including the frequency and the incidence of tropical cyclones and associated storm surge. Rainfall relates to run-off from the floodplains as well as water level within the main river systems. Winds, including those generated by tropical cyclones, affect waves, water levels at the coast and currents in the estuarine reaches of the rivers. Unfortunately, no wave statistics are available for the southeastern part of Van Diemen Gulf. However, the wave regime is unlikely to be a significant factor in the narrow riverine estuaries of the Gulf. In the sheltered waters of the Gulf, and especially because the shore is flanked by a broad, shallow sub-tidal terrace, the most significant processes affecting salt water intrusion of the coastal lowlands are those

related to fluctuation in sea level in response to changes in the wind regime. In this respect winds are a good proxy for wave and water level change.

The climate is strongly dominated by its seasonal character, most simply classified into wet and dry seasons. As may be expected from this description, there is a sharp difference in rainfall between the wet and dry seasons. Variation in rainfall, including rainfall intensity and the duration of the wet season (Fig 3), produces an immense change in the quantity of freshwater runoff transported across the Alligator Rivers catchments. In turn, this markedly affects the salinity structure of the estuaries, pushing the salt wedge seaward during the wet season when it overwhelms the potential marine ingress produced by elevated mean ocean water levels.

Figure 3 Rainfall at Oenpelli (Gunbalunya). Time series of total monthly rainfall (top).

Monthly rainfall variability (bottom).

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Wind observations at Darwin from 1950 to 1997 were used to examine the nature of the regional winds, including interannual variability of the monsoon. Prevailing weather conditions are dominated by the south-easterly trade winds during the dry season, between April and August, and the north-westerly monsoonal winds during the wet from September to March (Fig 4). The intensity of winds during 6-monthly periods incorporating the wet and dry seasons respectively is comparable with modal winds approximately 3.5 m/s and 5% exceedance winds approximately 7.5 m/s. The changing pattern of wind directions throughout the course of a year is relatively distinct, with easterly to south-easterly winds dominating in the dry and westerly to north-westerly in the wet (Winn et al 2006).

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However, high runoff during the wet season may erode new channels or extend channels that are subsequently inundated by tidal flows during the following dry season. Further, entrenchment of channels may occur in the wet at times when river flooding is coincidental with offshore cyclonic winds and nearshore water levels are unusually low.

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Figure 4 Wind direction, frequency and speed at Darwin from 1977 to 2004. Frequency of wind events recorded at Darwin between 1977 and 2004. The most frequent directions are those associated with the north-westerly monsoonal winds of the wet season. The variability of maximum wind speed is associated with the activity of tropical cyclones but is well below maximum gusts recorded for extreme events outside the period for which records were available for analysis. For example peak wind speeds of up to 235 km/h were recorded near Jabiru during Cyclone Monica in 2006.

The westerly to north-westerly winds produce water level set-up in the south eastern sector of Van Diemen Gulf. The set-up is expected to be highest under extreme, prolonged north-westerlies and in the vicinity of the South and East Alligator River (Green 1996). Although interpretation of the wind record is complicated by missing data, there is a gradual increase in northerly winds over the observation period (Fig 5). While more direct measurement of water levels within the Gulf is necessary to test the proposition, it is anticipated that persistent onshore winds would result in a significant increase, albeit slight, in water levels along the south eastern shore of Van Diemen Gulf over the same period. The significance derives from the close proximity of the floodplain elevation to spring high tide level.

Although strong winds may be associated with monsoonal activity in the wet (Fig 4), the extreme storms of the region are associated with highly mobile and intense tropical cyclones

that occur infrequently during the wet and early in the dry. Up to 129 tropical cyclones traversed the region within a 10o square between 1950 and 2003. A maximum of seven occurred between 1989 and 1990. Tropical cyclones may generate extreme winds from any direction depending on the cyclone path in relation to the coast and the relative intensity of the weather system.

Figure 5 North-westerly wind component of the wet season. The solid line is the annual data and the

dotted line the five year moving average.

2.2 Fluctuations in sea level Unfortunately, there are no permanent water level gauges in Van Diemen Gulf. Tide gauges that are closest to the Gulf and thus the Alligator Rivers Region are the Darwin SEAFRAME gauge, a high resolution gauge serviced by the National Tidal Facility (NTF); the Darwin tide gauge which was in operation from 1/12/1958 to 31/12/1994; and the Melville Bay Gove Island tide gauge. The nearest gauging station to the Alligator Rivers Region is the Darwin SEAFRAME gauge, which began recording in May 1990. Thus water levels within Van Diemen Gulf are expected to generally follow the patterns observed at Darwin. It must be noted that these patterns become less relevant for short-period water level signals, or further upstream within the Alligator River estuaries.

Darwin is located within a macro-tidal region, experiencing semi-diurnal tides. Tidal levels nominated by the Australian National Tide Tables (Department of Defence 2002) relative to Darwin Chart Datum are shown in Table 1.

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Table 1 Tide levels relative to Darwin Chart Datum

Highest Astronomical Tide 8.0 m

Mean High Water Spring 6.9 m

Mean High Water Neap 5.0 m

Mean Sea Level 4.1 m

Mean Low Water Neap 3.2 m

Mean Low Water Spring 1.4 m

Lowest Astronomical Tide 0.0 m

A seasonal tidal constituent, the sum of harmonic tidal components with periods greater than monthly, of 0.108 m has been calculated. This produces a range of approximately 0.2 m, peaking in February–March and lowest around July–August. It corresponds with the assessment of seasonal water level patterns around Australia (Pariwono et al 1986). In comparison with the tidal signals, tidal residuals are relatively small. Comparison of the monthly maximum residuals from 1995 to 2001 (from NTF) shows mild residuals of ± 0.2 m throughout the year. Whilst pressure systems contribute the majority of the residual, there is a small contribution due to mean sea level variation. More significant residuals occur during the wet season, with up to ± 0.6 m recorded over the short period of observation. The greatest residuals are most commonly related to tropical cyclone generated surge, whether by direct impact or shelf wave generation. It is considered likely that the range of residuals would increase markedly over a larger period, as the period is not representative of the entire range of tropical cyclones. In particular, passage of Tropical Cyclone (TC) Tracy in 1974 generated a tidal residual greater than 2.0 m.

Mean monthly sea level observations from 1959 to 2000 have been plotted in Figure 6. The mean annual water level varies by approximately 0.2 m, with a varying seasonal component. Linear regression of the data set suggests a trend of –0.02 mm/y for 1959 to 1997 (Mitchell et al 2001). The lowest mean level occurred in 1992 and the highest was observed in 2000.

Figure 6 Sea level at Darwin and the Southern Oscillation Index

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As discussed by Pariwono et al (1986), there is a strong relationship between the Southern Oscillation Index (SOI) and mean sea level. This is confirmed by the remarkable coherence between running means of Darwin monthly water level and the unscaled monthly SOI also shown in Figure 6. It should be noted that SOI holds a very strong relationship to the Darwin barometric pressure (at mean sea level), which therefore suggests a relationship between mean water levels and interannual movement of the tropical pressure belts. However, due to its latitude, Darwin is weakly influenced by the 18.6 year lunar nodical cycle. The main interannual tidal cycle is developed by the evolution of lunar perigee. This is expressed approximately as a 4.4 year cycle due to the strong semi-diurnal tidal character.

Sea level fluctuations facilitate estuary salinisation through influx of ocean water during raised water levels and efflux of estuarine water during lowered levels. This occurs over a range of time scales, including tidal cycles, storm surges, seasonal changes and longer fluctuations. The relative significance of such exchanges upon salinisation is determined by the degree to which mixing of estuarine waters occurs during each cycle and for each scale. Consequently, longer-period cycles typically have greater relative influence upon estuarine salinity.

The effect of short-period fluctuations upon salinisation is influenced by the salinity structure within the estuary and the relative degree of mixing that occurs during any water level excursion. Ryan et al (2003) suggest that for a tide-dominated estuary, high levels of mixing occur in the main tidal channel. In contrast, relatively low quantities of mixing occur in the tidal creek system. For very short-period fluctuations, such as those associated with semi-diurnal spring tidal cycles and surge events driven by the passage of tropical cyclones over several days, greater rates of mixing downstream than upstream determine that a negative surge is expected to produce a greater increase in salinity than a matching positive surge (Winn et al 2006).

Descriptions of tropical cyclone structure and character are available from a range of sources. Relatively early assessment of cyclone formation and character was provided by Gray (1975), with further detailed analysis of tropical cyclone characteristics by McBride & Keenan (1981). Cyclones’ effect in generating storm surge is partly dependent on their area of generation and the path followed as they move along the coast. Tropical cyclones may generate significant positive, negligible or negative surges according to whether their path is offshore, alongshore or over land respectively. In Figure 7 this is illustrated for storm surges in the Van Diemen Gulf and water levels recorded at Darwin. Another important characteristic is the timing of tropical cyclones in short and long-term contexts. Mid-season tropical cyclones occur at the same time as the wet season, and hence any effect the surge may have on estuarine salinity may be obscured by freshwater flow.

It is important to note that the degree of upstream mixing may be dramatically increased if an excursion extends upriver beyond the tidal creek network under consideration. Analysis of extreme water level events from the 1959–1997 Darwin data set shows that a strong majority of high water level events are associated with ‘king tides’, the highest astronomical tides plus local set-up, rather than tropical cyclones. This demonstrates the significance of mean sea level fluctuations. Nevertheless, tropical cyclones provide potential for truly extreme water levels and must be considered. The localised nature of such impacts underscores the point that the Darwin water level history cannot be directly used as a descriptor of extreme water level events in the Alligator Rivers Region.

Figure 7 Surges associated with tropical cyclones. The dots indicate hourly water levels at Darwin and the solid line results from application of a two-day triangular filter. Tropical Cyclone (TC) Les displays a

depressed water level; TC Max little departure from average; and TC Verna an elevated water level. The small inset shows the paths of the cyclones in relation to Van Diemen Gulf (from Winn et al 2006).

Estimation of a relative sea level fall of 0.02 mm/year has been made from a single tide gauge at Darwin over the period 1959 to 1997 (Mitchell et al 2001). Following Coleman and Wright (1978) and Kench (1999) from elsewhere, this is unlikely to be meaningful for the south eastern shores of Van Diemen Gulf due to differences in geology and the likely compaction of floodplain sediments within the Gulf. It indicates a requirement to establish differences in relative sea level change between coast abutting the horizontally bedded sandstones outcropping at the coast and the unconsolidated fluvio-estuarine silts and muds of the extensive coastal plains.

In addition, exchange may be significantly enhanced through relatively small changes of the mean sea level (Wolanski & Chappell 1995) or through change in the hydraulic character of the estuary channel (Vertessy 1990). For a typical tidal channel and basin system, a rise in mean sea level results in enhanced tidal exchange through two mechanisms. The increased depth covers a greater area, and hence provides a greater tidal prism. It also reduces the bed friction, allowing greater discharge. Whilst the latter may be significant in some cases, it is the former mechanism that holds the greatest significance, particularly for the semi-emergent

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wetlands or basins with shallow margins, characteristic of the Alligator River Region wetlands.

2.3 Morphology and landscape change on the coastal plains A significant amount of scientific research has been undertaken on the Alligator Rivers Region. It commenced in the early 1970s with the Alligator Rivers Region Environmental Fact Finding Study (Christian & Aldrick 1977). Results of that research subsequently were used in an assessment of the impact of mining and milling uranium ore by the Fox Inquiry (Fox et al 1977). Research has continued in the region to gain information for the management of the National Park.

2.3.1 Morphology A selection of landforms of the Alligator Rivers Region is illustrated in Figure 8. The geomorphology of the coastal plains of the Mary River is known from research by Woodroffe and Mulrennan (1993) and South Alligator River systems by Hope et al (1985), Woodroffe et al (1985a, b & c, 1986), the Magela Creek and coastal plains by Nanson et al (1990) and Wasson (1992), and the Point Stuart chenier sequence by Clarke et al (1979) and Lees (1987). General descriptions of landform evolution in the region have been provided by Story et al (1969), Christian and Aldrick (1977) and Duggan (1985, 1988). Detailed stratigraphic investigations of the South Alligator River system by Chappell and Grindrod (1985), Woodroffe et al (1986, 1989) provide evidence of major sea level and environmental changes in the region over the past 7000 years. These investigations provide a context for environmental changes currently occurring in the region as well as for the higher frequency changes that have occurred in the past 100 years and which may recur in the future.

Long-term changes in floodplain geomorphology, which take place over several thousand years, provide a context for several higher frequency processes that are likely to contribute to evolution of the floodplain and affect the balance of saline versus freshwater wetlands. A recent, dramatic change to the wetlands is due to incursion of tidal creeks (Fig 9) across the supratidal flats and into formerly freshwater wetlands (Fig 10). The rapidity with which the networks of tidal creeks has expanded and intensified during the past 50 years on the Mary River is indicative of either a trigger mechanism that has moved the floodplain system towards a new morphological state, or short-term fluctuations in atmospheric, fluvial and oceanographic processes.

Maximum elevations of the coastal floodplains of the Mary River and the Alligator Rivers Region are less than five metres, and commonly close to spring high tide approximately 3 metres above mean sea level. Substantial regions of the coastal plains are at elevations below this (Wasson 1992, Woodroffe & Mulrennan 1993). Many of the remote backwater plains lie at or below the elevation reached by the highest tides (Fig 8a), yet are protected from tidal inundation by the slightly higher elevation of levee-like features that lie adjacent to the river channels (Knighton et al 1991). The low gradient of the coastal and estuarine floodplains of the northern coast, with a seaward gradient of as little as 0.5 m over 70 km, emphasises the degree to which they are vulnerable to exploitation by invading saltwater channels, or are likely to become evaporative ponds in the dry, following overbank flooding during the wet season, or during phases of extreme spring tidal fluctuation (Fig 10).

Figure 8a Floodplain of the South Alligator River, south of the Arnhem Highway, in the foreground. Freshwater wetlands, back swamp basins adjacent to higher ground, are apparent in the central background of the photograph. Photo: Ian Eliot

Figure 8b Palaeochannel on the floodplain of the East Alligator River. Photo: Ian Eliot

Figure 8c The lower right foreground shows buffalo wallows in meadows adjacent to the Mary River. Linkage of palaeochannels through incursion of tidal creeks along buffalo tracks and wallows has been suggested as one cause of salt water intrusion into freshwater meadows. Photo: Ian Eliot

Figure 8d Pig diggings and wallows on the banks of a tidal creek near the mouth of the East Alligator River. Pigs are currently responsible for disturbance of the land surface that may result in acceleration of local erosion. Photo: Mike Saynor

Figure 8 Landforms of the Alligator Rivers floodplains

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Figure 9a Tidal creek crossing supra-tidal mudflats near the mouth of the East Alligator River. Mangroves line the channel and its tributaries. Shallow basins have formed on either side of the main tidal creek. Photo: Mike Saynor

Figure 9b Tidal creek crossing a mudflat near the mouth of the East Alligator River and invading a formerly freshwater wetland. Mangroves line the channel of the tidal creek. Photo: Max Finlayson

Figure 9c Tidal creek invading a formerly freshwater wetland. Mangroves line the channel of the tidal creek. The photograph shows the wetland under flood conditions. Photo: Max Finlayson

Figure 9d Dead trees in a back-swamp basin invaded by a tidal creek near the mouth of the East Alligator River. Photo: Mike Saynor

Figure 9 Tidal creeks in different landscape settings and a saline basin resulting from incursion of a tidal creek

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Figure 10a First in a sequence of three photographs showing tide water incursion of a back-swamp basin at Kapalga on the South Alligator River. An incipient tidal creek is apparent as a line of dried, cracked clays in the centre of the photograph. Dead trees line the creek and salt tolerant grasses are colonising its banks. Photo: Ray Hall

Figure 10b Second in the sequence of three photographs showing tide watincursion of the back-swamp basin at Kapalga. Salt water is visible at the surface of the channel and confineits banks. Photo: Ray Hall

er

d by

Figure 10c Third and final in the sequence of photographs showing tide water incursion of the back-swamp basin at Kapalga. Salt water is no longer confined by the stream banks and is beginning to spread into the basin. Photo: Ray Hall

Figure 10d Cracked clays on the bed of the dry tidal creek at Tidal Creek near Point Farewell – East Alligator. Photo: Kristy Winn

Figure 10 Tidal incursion of a back-swamp basin at Kapalga

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2.3.2 Processes Knighton et al (1992) suggested that several factors have contributed to the vulnerability of floodplains to extension of tidal channels. They noted that tidal channels develop through a combination of extension and widening of the main channels as well as through tributary growth. The process of tidal channel formation reportedly begins with overbank flooding of saltwater over the floodplains during exceptionally high tides (Knighton et al 1992). Wet season floodwaters may act to accentuate the process of tidal scour. Six metre spring tides in Van Diemen Gulf allow the effects of tidal action to occur at the headwaters of the tidal channels, up to 105 kilometres inland (Woodroffe et al 1986). Furthermore, the macro-tidal range ensures there are bi-directional currents with high velocities within the tidal influence of channels and hence a high potential for tidal scouring (Knighton et al 1991).

Distinct palaeochannels are recognisable within the Alligator and Mary River regions. These are remnant tidal channels that were active during the mid-Holocene, and have since been partially or completely infilled by the deposition of tidal mud and sediments (Woodroffe et al 1986; Woodroffe & Mulrennan, 1993). They are apparent as billabongs, freshwater swamps and wetlands (Fig 8b), and are therefore particularly vulnerable to saltwater incursion. As palaeochannels are generally some of the lowest-lying topography within a coastal floodplain, they act as low-land catchments for the development of seepage zones responsible for the initiation of channels (Woodroffe & Mulrennan, 1993). Subsequently, palaeochannels may be preferentially invaded by the expanding network of tidal creeks. Whilst sediment size data has generally been unconvincing in demonstrating this preferential invasion by the tidal creeks, Woodroffe and Mulrennan (1993) suggested that the alluvial deposits of palaeochannels should be more easily eroded than soils that have developed in situ. Given the erodability of the deposited sediments comprising the palaeochannels, they are generally associated with bordering levee banks of higher relative elevation. Subsequently, palaeochannels, once inundated, tend to confine the pattern of saltwater intrusion and form saline basins.

Processes driving change on the floodplains do not appear to be thoroughly understood. Primary mechanisms of floodplain adjustment to interdecadal variation in climate and sea level, and the coupling of oceanic and atmospheric processes are not well described. In particular, oceanographic processes in Van Diemen Gulf warrant closer examination as potential drivers of coastal change in the Alligator Rivers Region. In particular, Gulf water levels and water circulation respond to marked variation in climate through phases of pronounced ENSO activity occurring at sub-decadal and longer frequencies, including a 19 year tidal syzygy. These affect sea levels and are likely to be associated with local erosion around the south eastern shores of the Gulf and in the estuaries, precipitation in the stream catchments, river discharge regimes and channel sedimentation. All require further, more detailed, investigation that is beyond the scope of this paper.

In the absence of detailed meteorologic and oceanographic information and its interpretation, the impact of large numbers of uncontrolled feral buffalo on the erosion of tidal channels has attracted significant attention within the literature as a trigger mechanism for changes in floodplain geomorphology although it may be of secondary importance only. It is a commonly held view that buffalo grazing and trampling along swim-channels have hastened, if not initiated, the extension of tidal influences (Stocker 1970). From an examination of 1950 and 1981 aerial photographs, Fogarty (1982) suggested a correlation between the extent of saltwater intrusion and an increase in buffalo on the floodplains, although correlation should not imply causation. Similar observations have been made on the South Alligator River floodplains (D Lindner pers comm cited in Finlayson et al 1988). However, while buffalo tracking and wallowing may have had a significant effect on the landsurface (Fig 8c), it is

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hard to see how it would cause widespread effects along all the channels. This is particularly apparent where creeks are lined by wide mangals or in places where the tracks cut at right angles across the long axes of palaeochannels and subsequently formed tidal creeks. The proposition that invading tidal creeks tend to run along the long axis of palaeochannels whereas buffalo swim paths and tracks cut across them is one that warrants further investigation.

Although buffalo numbers were particularly high around the period when creek networks of both the Mary and South Alligator rivers began to erode (Woodroffe & Mulrennan 1993, Bayliss & Yeomans 1989), numbers have declined in recent years due to culling to stop the spread of brucellosis. Since the 1970s, and with the assistance of reclamation work, removal of buffaloes from areas of the South Alligator floodplain has allowed natural regeneration of some of the disturbed areas. Despite these actions, the establishment of dams and levees across tidal channels of the South and East Alligator Rivers have had varying success in preventing saltwater intrusion (D Lindner pers comm). This is an unsurprising outcome given that tributary (gully scour) and distributary (depositional fans) in the headwaters of tidal creeks indicate that some creeks are eroding salt flats and lowering the land surface whereas others are depositing sediments and raising it (Fig 11). The geography of deposition versus erosion is unknown but thought to be related to floodplain adjustment to relative change in sea level at an inter-decadal scale. Winn et al (2006) describe switching between the two states where tidal creeks have invaded a wetland basin near the East Alligator River.

Expansion of bare mudflats and the headward expansion of tidal creeks into freshwater meadows and ponds may be affected by a suite of secondary variables (Table 2). Salt water intrusion results in the death of freshwater vegetation and creation of bare surfaces susceptible to Aeolian erosion from the Gulf of Carpentaria, as has been pointed out by Rhodes (1980, 1982). During the dry season the sediment surface is smoothed by tidal flows but dries to desiccation and cracks deeply between periods of tidal inundation (Fig 10d) thus leaving the surface exposed to Aeolian processes. Dust storms were prevalent during the field investigations on the floodplains of Van Diemen Gulf and removal of fine sediment from the bare surfaces was observed during the strong south-easterly winds prevailing at the time. Sediment deflated from the surface was trapped by vegetation on the periphery of the bare mudflats and retained within the area. This is indicated by the presence of numerous micro-dunes, approximately 5 cm in height and observed close to and within patches of vegetation (Fig 12). Subsequently the microdunes may be removed by flood run-off during the wet. The significance of this secondary process in potentially lowering basin surfaces to level critical to tidal creek extension is not known although it is worthy of further investigation.

Figure 11a Floodwaters covering mudflats near the East Alligator River. Tidal creeks and their overbank distributary features are apparent in the foreground. A small tidal creek drains lowland between the levees of the two main creeks. Photo: Maria Bellio

Figure 11b Headwater tributaries of a tidal creek showing the detail of the dendritic tributaries formed by run off and gully erosion during high water conditions. Photo: Mike Saynor

Figure 11c Headwaters of a tidal creek at low tide. The main channel is approximately 10m wide. Crescentic patterns at the head mark inundations by the tide as it falls from spring to neap conditions and indicate the headwaters may be distributing water and sediment under dry season conditions. Photo: Mike Saynor

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Figure 11d Small depositional fan deposited by flood tide intruding into a saline basin at Kapalga on the South Alligator River. The fan is a small version of depositional fans formed on some parts of the mudflats and coastal plains (See Figure 11a). Photo: Mike Saynor

Figure 11 Morphologic variations at the headwaters of tidal creeks

Figure 12a Mudflats on the left hand side of the photograph are an apparent source area for the rippled loess sheets on the right hand side. Photo: Kristy Winn

Figure 12b The foreground shows ripples on a sand sheet traversing salt flats at Kapalga during the Dry season. Photo: Ian Eliot

Figure 12c Loess, fine silt and clay, may be blown from mudflats and basins in the dry season. The photograph shows a layer of loess covering cracked clays on the margin of a basin near the mouth of the East Alligator River. Photo: Kristy Winn

Figure 12d Micro-dune formed in the lee of vegetation on the margin of a floodplain basin. Photo: Kristy Winn

Figure 12 Wind transport of sediment may play a minor role in deflating and lowering of dry basins during the dry season

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Table 2 Relationships between the primary and secondary variables (after NCCOE 2004)

Primary variables

HABITATS

(Secondary variables)

Mean sea level Severe weather systems

Trade winds & monsoonal winds

Rainfall & runoff Wave climate Isostatic change

Seagrasses and inshore biota

Projected change in depth is small for the effect upon light penetration and water temperature.

Changes in turbidity associated with increased storminess may affect light penetration; episodic erosion of sea grass banks may occur with increased storm intensity and/or duration.

Mixing and turbidity may affect light penetration.

Localised salinity changes may occur in nearshore waters, particularly near stream mouths.

Changes in turbidity and sediment transport on the sea bed, as well as episodic erosion may occur with increased storm intensity and/or duration.

Potential change due to floodplain settlement and compaction is negligible.

Nearshore environments & sandy beaches

Adjustment of zonation in response to shoreline recession and erosion, as well as change in sea level.

Balance of erosion versus deposition alters with change in frequency, intensity and duration of onshore weather conditions.

Balance of erosion versus deposition alters with intensity and duration of onshore winds.

Changes in groundwater state of sandy beaches may contribute to rates of erosion.

Balance of erosion versus deposition alters with change in frequency, intensity and direction of onshore weather condition.

Potential change due to floodplain settlement and compaction is small.

Mangrove levees on major rivers & streams

Major changes in zonation of mangal communities; potentially large changes in salinity structure due to increased tidal exchange.

Possible defoliation due to wind stripping, sediment movement and salt encrustation. Active cheniers moved landwards.

Sediment movement due to wind effects on local wave climate.

Levee destabilisation due to flooding; basal sapping of banks on tidal creek; and movement of the saline wedge.

Affects near coastal areas; minor effects on the stability of stream banks.

Potential change due to floodplain settlement and compaction is small.

Tidal creeks & mangroves

Potentially large change in zonation; increased tidal exchange and salinisation.

Short term changes in flow patterns due to storm surges.

Secondary effects due to wind set-up of sea level.

Scouring of tidal creeks by freshwater run-off during the wet season; salinity gradient is re-established during the dry.

Not applicable. Extension of tidal creek networks and mangrove colonisation upstream.

Mudflats and salt-tolerant wetlands

Potential increase in the area of mudflats and salt tolerant meadows

Low lying coastal mudflats inundated by major floods and storm surges.

Movement of sediment by winds during the dry season.

Extensive inundation of lowlands by river floods.

Not applicable. Expansion of mudflats adjacent to the ocean, major rivers and tidal creeks

Freshwater basins and wetlands

Possible encroachment by mudflats and saline wetlands.

Extensive inundation of lowlands by river floods.

Movement of sediment by winds during dry season

Extensive inundation of lowlands by river floods.

Not applicable. Invasion of palaeochannels and freshwater ponds by tidal creeks.

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2.4 Stream hydrology The Alligator Rivers Region is drained by the South Alligator and East Alligator rivers with the smaller West Alligator and Wildman rivers draining the north-western portion of the region (Fig 2). The rivers are fed by a network of ephemeral creeks and drain into Van Diemen Gulf in the north. The combined catchment area of the four major rivers is approximately 28 000 km2. There is no significant groundwater mining in the region, which is largely under natural vegetation or exotic pastures. In describing the surface hydrology of the region, reference is commonly made to three of the major physiographic land surface units: the plateau and escarpment; the lowlands; and the floodplains (East 1996). Much of the information on the hydrology of the region comes from Chartres et al (1991), Kingston (1991), Nanson et al (1990) and Roberts (1991) and is summarised by McQuade et al (1996). Most recently, descriptions of the hydrology have been compiled by Walden (2000) and also by Moliere (2007) for the broader tropical rivers region.

Investigation of the hydrological record of the Alligator Rivers Region and analysis of the available information was undertaken as a first step to establishing a coastal monitoring program (Eliot et al 2000). As part of these investigations Walden (2000) noted that stream gauging stations have been deployed throughout region since 1957, and identified 39 gauging sites in rivers and along the coast of the region as being of use to the project. Despite the number of established stations, the hydrology of the Alligator Rivers Region is incompletely defined due to the highly variable and dynamic nature of its climate. The understanding and modelling of surface hydrology in the Alligator Rivers Region is made difficult by factors such as the high interannual variability in the rainfall, a lack of data describing variation in climate, and a poor knowledge of surficial sediment characteristics and distribution. Discharges at the mouths of the major rivers have not been systematically measured because tidal conditions make hydraulic rating extremely difficult (McQuade et al 1996). Additionally, extreme flood events simply overwhelm vast areas of floodplain, making it difficult to distinguish between the river mouths and the ocean (Fig 13). Despite this difficulty, estimated total annual flows at the mouths of the South Alligator and East Alligator Rivers are 2730 and 2560 million cubic metres respectively (Christian & Aldrick 1977). However, subsequent observations indicate these estimates considerably underestimate potential discharges during extreme events (Erskine & Saynor 2000).

Under all but extreme rates of discharge the river flow interacts with the tide. Chappell and Woodroffe (1985) and Chappell (1988) noted from hydrologic data of the Daly and South Alligator Rivers that the morphological differences of the channel segments appear to coincide with different tide versus flood discharge relationships. They identified four morphological components of the estuarine reaches of the two rivers (Fig 14). In downstream order these include upstream riverine, cuspate meander, sinuous meander and river mouth funnel components. The transition area of an estuarine channel where the sinuous or cuspate meanders meet the upstream segment was determined to be near the point where mean annual flood peak discharge (downstream flow) approximately equals the peak upstream flow or dry season spring tides (Chappell & Woodroffe 1985). Downstream, the transition from the funnel to the sinuous meanders of the channel occurs where the peak spring tide flow is roughly ten times the mean annual flood peak discharge (Chappell & Woodroffe 1985). The sinuous section of the river is the locus of active meander migration, where freshwater flood discharges exceed the flood tidal flows. Within the cuspate meander segment, however, the flood tidal flow is roughly equal to the seasonal freshwater flood discharge (Vertessy 1990).

Figure 13d The high-tide shore, mangal, saline grassland and mudflats near South Alligator River. Photo: Mike Saynor

Figure 13 Floodwaters and supratidal mudflats on the coastal plains between the East and South Alligator Rivers

Figure 13a Floodwaters covering mudflats in SE Van Diemen Gulf during an extreme flood event in 1998. The lines of vegetation across the central part of the photograph are mangroves growing along debris lines at the modal storm surge and flood inundation level. Photo: Mike Saynor

Figure 13b Floodwaters covering mudflats and mangals near Point Farewell in SE Van Diemen Gulf during an extreme flood event in 1998. The line of vegetation across the central part of the photograph is approximately at the high tide shoreline under dry conditions. Photo: Mike Saynor

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Figure 13c Floodwaters covering mudflats near the mouth of the South Alligator River during a small flood event in 2002. Two debris lines are apparent in the foreground, landward of the inundated mudflat. First is a line of logs and other flotsam, while further landward is a discontinuous line of mangroves growing along a second debris line. Photo: Maria Bellio

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e

Figure 14a Upstream, tidal river reach of the East Alligator River. The nomenclature broadly follows that of Woodroffe et al (1987). Riparian vegetation includes a succession of mangroves along the river bank, rainforest and freshwater meadow with distance away from the river channel. Photo: Ian Eliot

Figure 14b Cuspate meanders of thEast Alligator River. The nomenclature follows that of Woodroffe et al (1987). Photo: Ian Eliot

Figure 14c Sinuous meanders of the East Alligator River. The nomenclature follows that of Woodroofe et al (1987). Photo: Ian Eliot

Figure 14d The estuarine funnel at the mouth of the East Alligator River. The nomenclature follows that of Woodroffe et al (1987). A tidal creek lined with mangrove vegetation crosses the floodplain in the foreground of the photograph. Photo: Mike Saynor

Figure 14 Planform variation in channel morphology in the estuarine reaches of rivers in the Alligator Rivers Region (after Chappell & Woodroffe 1985 and Chappell 1988)

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2Spatial variation in the distribution of mangroves along the estuarine river systems of the Northern Territory coastline has been investigated relatively extensively within the literature,

ented, and relationships between

& Guppy 1988, Woodroffe et al 1989,

entify past and present changes in the sub-

The objectives of the research are to:

gion for each set of available aerial manner consistent with that used by

ning area of the Mary River floodplains;

.5 Changes in mangrove distribution

with broad-scale mapping of much of the coastline documthe spatial distributions and environmental factors identified (Hegerl et al 1979, Wells 1985, Davie 1985, Finlayson & Woodroffe 1996). Past research has documented supportive evidence of a close relationship between mangroves and sea-level fluctuations (Woodroffe et al 1987, Ellison & Stoddart 1991, Ellison 1993, Semeniuk 1994) and mangrove sediments have been identified as indicators of former sea level. Radio-carbon dating of mangrove wood fragments of the South Alligator River has been indicative of sea level changes dating over the past 6000 years (Woodroffe 1995). This recognition has lead to suggestions that the structure and distribution of mangrove communities may be associated with the tidal creek extension of saltwater influence into freshwater wetlands of the Alligator Rivers Region (Woodroffe et al 1986, Bayliss et al 1997).

Stratigraphic evidence on the coastal plains of the South Alligator, Adelaide and Mary Rivers indicates that in the last few thousand years the freshwater wetlands have replaced extensive mangrove forests to form the floodplains (Clarke Woodroffe & Mulrennan 1993). Preliminary radiocarbon and thermoluminescence data, based on the existence of shallow mangrove sediments and shoreline deposits in the stratigraphic record, indicate the coastal plains began a period of intensive progradation, with episodic chenier formation around 6000 years BP. The most rapid deposition occurred from 5000 years to 3000 years BP, with little depositional activity since 2000 BP (Woodroffe et al 1986, Clarke & Guppy 1988, Woodroffe & Mulrennan 1993). The change from a mangrove dominated saltwater regime to the extensive freshwater grasses and sedges of the coastal plains present today occurred sometime over the period of progradation. Although the plains of northern Australia have changed markedly over the last 6000 years of the Holocene (Russell Smith 1985, Clarke & Guppy 1988, Woodroffe et al 1986, Knighton et al 1991), there has been evidence of recent dramatic changes of the tidal creeks within the Alligator Rivers Region during the historical period.

More recently Landsat multi-spectral scanned (MSS) and Thematic Map (TM) imagery has been used to quantify short and long-term land cover changes on the Mary River floodplain (Ahmad & Hill 1995, Bach & Hosking 2002) to idsurface hydrology associated with saltwater intrusion (Jolly & Chin 1992). Additionally, Bell et al (2001) used AirSAR (Airborne Polarimetric Synthetic Aperture Radar) to map soil salinity, which is a key biophysical parameter for monitoring and managing environmental changes associated with saline intrusion. Despite extensive research conducted on floodplains of the Mary River and the South Alligator River, there has been no single causal explanation identified to account for the extension of tidal influences over the past 50 years. The recent changes are of immediate interest in research reported below.

3 Research objectives and methods

1 map the tidal channels of the Alligator Rivers Rephotography (1950, 1975, 1984 and 1991) in a Knighton et al (1992) for the adjoi

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3 have occurred

4 raphic processes likely to cause salt water incursion

5 th

The n two phases. Firstly, aerial photographs and topographic maps r

man was combined

ildman, West, South and East Alligator Rivers of the Alligator Rivers Region, was reconstructed from aerial photographs

lown during

2 determine the rate and spatial extent of tidal creek expansion and mangrove encroachment within the Alligator Rivers Region;

where appropriate, compare changes in the Alligator Rivers with those that elsewhere around the southern shores of Van Diemen Gulf over the same time period;

identify meteorological and oceanogof the coastal plains; and

describe morphology in the vicinity of the headwaters of tidal creeks associated wihydrodynamically different sections in the estuarine reaches of the rivers.

research was conducted iwe e interpreted to reconstruct and map patterns of tidal creeks and the distribution of

groves in the Alligator Rivers Region. Analysis of the aerial photographywith on-ground surveys to verify mapping of the most recent patterns observed on the aerial photography. The maps compiled were then used to describe topological properties of the creeks and estimate rates of changes observed for creek extension and mangrove distribution. At the time the data analysis was undertaken the 1991 aerial photography was the most recent available. Since then other remotely sensed imagery has become available including additional aerial photography in 2004. However these are not discussed in this report. The interpretations made herein, provide a base from which more recent (post 1991) changes to the tidal creeks and estuarine rivers may be assessed. Secondly, exploratory field mapping and general morphological descriptions were conducted at three sites – Point Farewell, Munmarlary and Kapalga – since these were accessible without substantial logistical support. All field work was undertaken over eight days during the month of August 1998 in the dry season. Descriptions of these steps have been outlined below.

3.1 Changes to the distribution of tidal creeks and mangroves Tidal creek expansion and mangrove encroachment along the W

for the years 1950, 1975, 1984 and 1991 (Table 3). The aerial photographs were fthe dry season of each year (May or June). This aided determination of the tidal reach of the creeks, as the creek tidal flows dominate over the freshwater floodwaters at this time. Although the photographs were seasonally consistent (Table 3), they varied in scale and quality. Mapping of tidal creeks was standardised using the method described in Knighton et al (1992) to overcome any differences. Maps of the tidal creeks and mangroves were drawn at the same working scale of 1:100 000 so that overlays could be prepared to enable comparison of topologic properties of the tidal creeks and the spatial distribution pattern of mangroves.

Table 3 Information on aerial photographs used in the interpretation

Date of Photography Scale Height (m) Colour Quality *

May 1950 1:50,000 7620 black & white 3

June 1975 1:25,000 3810 colour 1

May 1984 1:25,000 Undefined colour 2

May 1991 1:25,000 3962 colour 1

* Quality is defined in an arbi according e ease with which cree be identified, w is the easiest trary ordinal scale to th ks could here 1

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3.1.1 Network properties Geometric and topologic properties of the Wildman, West, South and East Alligator Rivers, were determined as measures of the rate of mangrove growth and the modes of tidal creek extension from 1950 to 1991. Network magnitude is a topologic variable corresponding to the number of exterior links of first order channels (Shreve 1967, Schumm et al 1987), and was used as a measure of network size. The area of mangroves was calculated from the maps constructed for each year (1950, 1975, 1984 and 1991) as a measure of the rate of mangrove encroachment. The trends of network growth and mangrove expansion were depicted in graphical format for each river system and related to the spatial distribution patterns.

3.1.2 Field surveys Three field sites, Point Farewell, Munmarlary and Kapalga (Fig 15) were selected on the basis of local knowledge of areas exhibiting evidence of saltwater intrusion and site accessibility. The latter was important given time and practical restrictions on fieldwork in the remote areas of Kakadu National Park. However, field observation of similar locations on the other river systems of the region indicate a degree of similarity that should be verified by future surveys. Tidal creeks and their immediate surrounds at each site were mapped in detail as a baseline for future measurement and assessment of geomorphic change. The three sites are described as follows:

3.1.2.1 Point Farewell First, the site at Point Farewell is located within the estuarine funnel on the southern bank of the East Alligator River, approximately 2 km south-east of Point Farewell (Fig 15). It was selected because of the occurrence of saltwater intrusion on freshwater meadows and extensive die back of Melaleuca spp.

3.1.2.2 Munmarlary Second, Munmarlary is in an area transitional between the estuarine funnel and the sinuous meanders of the South Alligator River, 30 km upstream from the river mouth (Fig 15). There is evidence of extensive growth of tidal creeks and mangrove encroachment within the confines of a palaeochannel swamp on the western flank of the river. Accessibility to the palaeochannel was impractical given logistical difficulties and time restraints on field work. Hence field mapping was conducted at the tidal creek to the east of the river. Aerial photographs taken in 1950 and 1991 indicate the creek remained morphologically stable, although mangroves have since colonised it.

3.1.2.3 Kapalga Third, Kapalga is located on the cuspate meander segment of the South Alligator River, approximately 40 km upstream from the river mouth (Fig 15). It was selected because of its accessibility and because a single tidal creek was known to have cut into a former freshwater swamp. There was also evidence of dieback in the Melaleuca spp forest. The site was revisited over a number of days and the area extensively mapped.

The main morphological components of the field sites were mapped using an ASHTECH differential Global Positioning System (dGPS) to provide an accurate record of the boundaries of present site morphology. Features of each site were mapped using the Reliance rover of the dGPS with a portable Marine IV antenna. The banks of the tidal creek at the Kapalga site were mapped using the rover receiver mounted on a boat during a spring high tide of seven metres. This enabled travelling as far upstream as possible without getting stranded as the tide turned. Other features were mapped with the receiver carried in a backpack.

Features of each site that were mapped included the mangrove boundary flanking tidal creeks as an indicator of the extent of saline influence, areas of salt flats, patches of dead Melaleuca, boundaries of Melaleuca forest, tidal channels, single mangroves, areas of dead mangroves and areas of mangrove seedlings. A five second logging interval was set on the dGPS for line and area features, and a 20 second logging interval for point features. Field observations and photographs were recorded in order to document features of the site. This information included notes on the vegetation types and general distribution within the sites, soil structural characteristics and surface cover. Surface soil samples were taken from Kapalga and Point Farewell for analytical laboratory analysis (electrical conductivity from the methods of Rayment and Higginson 1992). Sites where soil samples were taken were also mapped using dGPS.

Figure 15 Location of the field study sites and dGPS maps of the morphology at Point Farewell,

Kapalga and Munmarlary

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The dGPS data logged in the field were down-loaded daily from both the base station and the rover receiver to the Reliance software package. The down-loaded field data were then processed using the Reliance software, which converted the corrected data to a graphical format. Logged information included recordings of latitude and longitude values, elevations, and measurements of accuracy. The processed data were exported and manipulated into Arc Info and Arc Edit to generate maps of each field site.

4 Distribution of tidal creeks and mangroves: 1950 to 1991 Since the late 1940s to early 1950s, the tidal influence of estuarine rivers in the Alligator Rivers Region has extended along creek lines and the resultant changes in the saltwater reaches have been coupled with expansion of the area colonised by mangroves. The trends of change identified from the aerial photography are separately described for each of the rivers.

4.1 Wildman and West Alligator River The Wildman and West Alligator Rivers lie within the western flank of Kakadu National Park (Fig 2). Since 1950, tidal creeks of both rivers have exhibited marked changes in the distribution of mangroves along their banks. Mangrove encroachment has occurred along the main tributaries of the West Alligator and Wildman Rivers and along smaller creek lines as they apparently became more tidally active (Figs 16 & 17). Mangroves densely flank the shoreline of both river systems, with colonisation becoming sparser in the upper reaches of the rivers. The upstream estuarine segments of both rivers had a limited distribution of mangroves, although from 1950 to 1991 mangroves had extended approximately four kilometres upstream on the West Alligator River (Fig 17).

4.2 South Alligator River From 1950 to 1991, existing tidal creeks of the South Alligator River developed though a combination of headward extension and growth in the number of tributaries (Fig 18). In 1950, tributary development was relatively limited in the sinuous, cuspate and upstream sections of the river, whilst the most extensive network of tributaries was within and extending from a large palaeochannel on the eastern flank of the estuarine funnel.

By 1975, the tidal creeks which appeared relatively inactive in 1950 had extended in the middle and upper reaches of the South Alligator River. A number of creeks successfully invaded the series of palaeochannels flanking the sinuous and cuspate meandering segments of the river. Small creeks also extended from the main channel in the fluvial reach.

Tributary development in the estuarine funnel was limited relative to the changes occurring on the other river segments. From 1975 to 1991, tributary growth remained active within the confines of the palaeochannel boundaries along the meanders of the South Alligator River, whilst they remained fairly limited elsewhere. The significant contribution of small topographical features to formation of the network of tidal channels is evident from this phase of growth.

Expansion of the tidal influence along extending creek lines of the South Alligator River is indicated by spatial changes in the distribution of mangroves along the banks. In 1950, mangroves were distributed along the main tributaries of the South Alligator River (Fig 18). They had extensively colonised the shoreline and main channels of the estuarine funnel. At this time mangroves were distributed in patches along the main tributaries in the sinuous and cuspate segments, and few mangroves colonised the upstream segment of the river. By 1991,